In the state of the art process for preparing poly(vinyl alcohol), two azeotropic mixtures are formed downstream of the poly(vinyl acetate) reactor which are very difficult to separate using conventional techniques. These two azeotropic mixtures are (1) methanol/vinyl acetate and (2) methanol/methyl acetate.
The prevailing method to break these two azeotropic mixtures is a water extractive distillation method wherein water is used to wash the methanol out of the mixture to yield a relatively pure acetate organic phase at the overhead and a methanol/water phase at the bottom in an extractive distillation column. The methanol/water non-azeotropic mixture is then separated by simple distillation with the water being recycled to the extractive distillation column. For example SU 1313-849-A suggests a methanol to water ratio of 1:3-3.2 for a feed of vinyl acetate, methanol and some acetaldehyde and methyl acetate. The overhead is further phase separated and purified to reduce volatiles and water carryover.
DD 241-253-A, on the other hand, suggests an azeotropic distillation of methyl acetate and methanol and/or water by using a boiling hydrocarbon fraction in addition to adding water to yield high recovery of methyl acetate.
U.S. Pat. No. 4,549,938 suggests an extractive distillation method for methanol/methyl acetate mixtures by adding higher boiling oxygenates such as ethylene glycol ether, ethylene carbonate, nitromethane, etc.
Both JP 54-135673 and JP 53-45677 suggest the use of membranes to separate the methanol/methyl acetate azeotropic mixture.
U.S. Pat. No. 5,026,482 suggests a concentration swing adsorption process for methyl acetate/methanol separation in which ethanol is used to regenerate the adsorbent. The subsequent ethanol/acetate and ethanol/methanol mixtures are then separated by simple distillation.